1. Field of Invention
This invention relates to a fully hydraulic power steering system, particularly suited for large vehicles. This system is of the type using a power pump for normally supplying working fluid to a pair of steering motors and a manually operated metering pump for controlling positioning of the steering motors.
This invention also relates to steering systems having a variable ratio for allowing the operator to generate sufficient fluid pressure to control the steering system in the absence of fluidized pressure generated by the power pump.
2. Description of the Related Art
Power steering systems having a manually operated steering wheel for generating fluid pressure at a metering pump which in turn controls a power pump for actuating steering are well known. U.S. Pat. No. 3,584,537 shows a constant assist hydrostatic steering system wherein an operator controlled metering pump directs fluid to a secondary motor for metering movement of the steering system and controls fluid delivery from a motor driven hydraulic pump to a primary steering motor. Another dual servomotor system is shown in U.S. Pat. No. 3,657,888, wherein manually generated fluid pressure drives one servomotor and operates a first and second control valve for regulating power assist to a second servomotor. In U.S. Pat. Nos. 3,554,089 and 4,028,997 a metering pump of the gerotor type and one or more motor-driven power pumps deliver pressurized fluid to a hydraulic steering system, wherein a first control valve regulates delivery of metered fluid flow to a first servomotor as well as delivery of power fluid to the intake of the gerotor while a second control valve arrangement responsive to pressure differential across the first servomotor regulates delivery of power fluid to a second servomotor. U.S. Pat. No. 3,765,181 expands the teachings of U.S. Pat. No. 3,554,089 to incorporate means in the second control valve arrangement for delivering pressurized fluid from the gerotor to the second servomotor in the event of failure of the power pump in order to achieve a change in ratio which will allow the operator to control the steering system without power assistance. Thus, the prior art has shown how to combine the convenience and comfort of a power assist steering system with the improved control of a full flow metering type system. Such systems also provide for dual steering ratios through dual servomotors which will allow the operator to control this steering during emergencies when power assist is not available.
While the metering systems of the prior art can provide accuracy and ease of operation, it is also desirable to have the steering system sensitive to variable operating conditions of the vehicle. For example, a greater degree of steering control is necessary when the operator is making small adjustments in vehicle direction or the vehicle is traveling at high speed. The need for such adjustments, usually occurs when the vehicle is traveling in a straight ahead direction. In the case of industrial or agricultural vehicles, precise straight ahead control greatly benefits its operation. It is known that a greater degree of steering control can be obtained by increasing the turning resistance at the steering wheel of a vehicle. This selected increase of turning resistance facilitates operator control without sacrificing operator comfort.